Because of its excellence in various properties such as heat resistance, durability, mechanical properties and surface properties like hardness, a thermosetting resin such as phenolic resin, epoxy resin and urea resin finds application in a wide range of fields such as electrical industry, electronic industry, automobile industry, construction industry and civil engineering as molding material, film-forming material, binder material, etc. With the enhancement of the requirements for these materials, extensive studies have been made to improve the properties of thermosetting resins by compounding.
In particular, compounding with an inorganic material as a reinforcement is one of the most widely practiced method for drastically enhancing various properties such as mechanical properties, heat resistance and electrical properties of a polymer. Examples of inorganic materials commonly used for compounding include fiber materials such as glass fiber and carbon fiber, and powdered materials such as calcium carbonate, silica, titania and magnesium oxide.
In practice, one or more of these inorganic materials are dispersed or mixed in a resin by melt blending method or solution blending method before use. In compounding, it is important to disperse or mix the inorganic materials thus added in the resin homogeneously as much as possible without causing ununiform agglomeration thereof to enhance the desired physical properties and hence obtain a composite material having a high performance.
On the other hand, it has been studied to use an inorganic material in a smaller form such as fine particle to accomplish further enhancement of physical properties. In general, however, the more fine particle the inorganic material is, the more difficultly can be made homogeneous mixing or dispersion thereof and the more strictly is restricted the enhancement of properties thereof.
Further, the more fine particle the inorganic material is, the higher is the price thereof. At present, it is the most usual that an inorganic reinforcement having a particle diameter of around 10 .mu.m is used. An inorganic reinforcement having a particle size of from 1 to a few micrometers is used depending on the purpose. Some inorganic materials are used in a particulate form as small as not more than 1 .mu.m.
It is thus thought that if inorganic materials having various sizes can be homogeneously and easily dispersed or mixed in a resin in a desired proportion, a new potential will be developed for a composite of an organic polymer with an inorganic material.
On the other hand, as a process for the preparation of a metallic oxide such as silica and titania there has been reported a method employing hydrolysis and polycondensation of a metal alkoxide, i.e., so-called sol-gel processing. In recent years, a method has been studied which comprises effecting sol-gel processing in an organic polymer solution to disperse the resulting metallic oxide in a polymer.
To date, as an organic polymer to be used as a matrix there have been mainly studied water-soluble polymers and some thermoplastic polymers. For example, water-soluble polymers such as polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone, polyoxazoline and sodium polystyrenesulfonate, polyoxytetramethylene, polyether ketone and (meth)acrylic polymer, modified hydroxyl-containing silicone resin such as polydimethylsiloxane terminated by a silanol group at both ends thereof, silicone elastomer, etc. have been studied to enhance the physical properties of various resins.
On the other hand, referring to thermosetting resins, a composite of a polyimide with silica obtained by compounding a polyamic acid as a polyimide precursor with a silicon alkoxide has been mainly studied (see "Journal of Material Chemistry", vol. 2, 679 (1992)). When its silica content is increased, the polyimide/silica composite obtained by using a polyamic acid becomes opaque, making it difficult to provide free control over the particle diameter of silica thus compounded. Further, when observed under a scanning electron microscope, most interfaces are observed without any adherence. Thus, despite its transparency, the composite exhibits an insufficient adhesivity at interfaces.
Moreover, the mechanical properties, except elastic modulus, of the composite thus obtained are greatly poorer than those of resin. This means that even a thermosetting resin/metallic oxide composite obtained by a process which comprises reacting a metal alkoxide in a polymer solution to produce a metallic oxide as well as render the matrix polymer heat-infusible cannot easily provide a high performance material having excellent mechanical properties. The use of a product of chemical modification of a polyamic acid, e.g., by the introduction of an ethoxysilyl group or the use of phenyltriethoxysilane as a starting material of metal alkoxide has been studied to enhance the mechanical properties of the polyimide/silica composite. However, these approaches can provide an enhancement of the elastic modulus of the composite at most but cannot attain a great enhancement of the strength and elongation of the composite as compared with the resin. Further, the method involving the chemical modification presents a cost problem and is hardly employed in practical use.
No reports have been made on studies of compounding with various thermosetting resins such as epoxy resin, phenolic resin and unsaturated polyester resin except for the use of shellac resin, which is a natural resin whose structure is indefinite (JP-A-5-65418 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")), and the use of a ketone resin (JP-A-5-331353). No examples have been reported, including the foregoing examples, of great enhancement of mechanical properties such as strength, elongation and elastic modulus of the composite as compared with the resin.